During the manufacture of semiconductor memories, such as synchronous dynamic random access memories (“SDRAMs”), it is necessary to test each memory to ensure it is operating properly. Electronic and computer systems containing semiconductor memories also normally test the memories when power is initially applied to the system. A typical SDRAM includes a number of arrays, each array including a number of memory cells arranged in rows and columns. During testing of the SDRAM, each memory cell must be tested to ensure it is operating properly. In a typical prior art test method, data having a first binary value (e.g., a “1”) is written to and read from all memory cells in the arrays, and thereafter data having a different binary value (e.g., a “0”) is typically written to and read from the memory cells. A memory cell is determined to be defective when the data written to the memory cell does not equal that read from the memory cell. As understood by one skilled in the art, other test data patterns may be utilized in testing the memory cells, such as an alternating bit pattern “101010 . . . 0” written to the memory cells in each row of the arrays.
In a typical test configuration, an automated memory tester is coupled to address, data, and control buses of the SDRAM, and applies signals to these buses to perform the desired tests. As the storage capacity of SDRAM and other memory devices increase, the number of memory cells and hence the number of data transfer operations the tester must perform correspondingly increases. For example, in a memory array having n rows and m columns of memory cells, the tester performs n*m cell accesses in writing the first binary data values to all the memory cells in the array, and thereafter performs n*m cell accesses in reading the same data. The tester must once again perform n*m accesses in writing data having a second binary value to each memory cell, and the same number of accesses in reading this data. The tester thus performs a total of four times n*m cell accesses, each of which requires a bus cycle to perform.
Data compression has been used by some testers to reduce the number of bus cycles required to test memory cells. Data compression generally relies on some means of quickly writing data to the memory cells of the memory device, and then reducing the amount of data that must be read from the memory device to indicate a pass or a fail condition. For example, sense amplifiers of an SDRAM device may be held at a particular logic level, such as a level corresponding to a binary “1” value, and the rows of memory cells sequentially activated, thereby quickly writing a binary value of “1” to each of the memory cells in the array. When data is read from the memory device, the binary values from all of the memory cells or groups of memory cells can be applied to an AND gate or other logic circuit. The logic circuit outputs a logic “1” if all of the memory cells in the row properly function to store the correct binary value. A similar process can then be used to write a binary value of “0” to all of the memory cells and then read the values stored in the memory cells. The results of reading each row can then be combined by conventional means so that the memory device will output a single binary value indicating either a pass or a fail condition.
In addition to compressed data testing of memory, semiconductor memory systems typically include logic to detect errors internally for correction. In general, error detection logic circuits are used to maintain the integrity of data stored in the memory array. When data are written to or read from memory, a calculation is performed on the data to produce or identify an error correction code (“ECC”). Both the code and data are stored, and the code is used to detect errors when the read data is compared to the data previously written. One type of error detection code generates parity bits for each data word written to memory. The parity bits are chosen so that the sum of the data and the parity bits are even. When the data word and parity bits are read from memory, the sum of the data word and the parity bits is determined. If the sum of the data word and the parity bits is an even number, then no error is detected. If one of the parity bits or one of the bits of the data word is in error, the sum of the data word and parity bits is an odd number. Although parity bits can be used to determine if the read data word contains an error, parity bits alone cannot be used to correct the error. A series of equations are derived using the parity bit configurations for generating ECC check bits particular to the data word being written to memory. The ECC check bits are written along with the data. When the data and check bits are read, the ECC logic detects the error by comparing the ECC check bits read from memory with ECC check bits calculated from the data read from memory. If no errors are detected, the read data are output from the memory device. If an error is detected, the ECC logic uses the read ECC check bits to correct the read data, and the corrected read data are output from the memory device. If too many bits of the read data are in error to be corrected, then the condition may be reported.
Hamming codes are conventional ECCs commonly use to detect and correct bit errors in a data word. The number of ECC bits for Hamming codes is determined by the total number of bits in the data and by the number of data bit errors that can be corrected. For example, an eight data bit word typically requires four check bits to correct a single bit error in the word, creating an ECC encoded word 12 bits in length. A conventional 8,4 Hamming code having eight data bits D0-D7 and four ECC bits P0-P3 is shown in FIG. 1A. To generate an ECC bit, the cells in that particular row having an assignment of “1” are x-or'ed together. All positions associated with each cell having an assignment of “1” are combined to define the parity equation for that particular ECC bit. For example, the equation for ECC bit P0 is determined by all positions having an assignment “1” in the first row. As a result, the equation for ECC bit P0 includes positions D0, D1, D3, D4, and D6. The equations for all ECC check bits P0-P3 are defined in the same manner, using the 8,4 Hamming code matrix of FIG. 1A, and are shown in FIG. 1B.
FIG. 2 illustrates a memory system 250 that may include the conventional Hamming code error correction scheme described above. The memory system 250 includes a memory array 270, a write bus 243, and a read bus 233. Input buffers 249 and input registers 248 receive eight bits of write data DQ<0:7>. Write FIFO 246 check and control the write data before transmitting to an ECC encoder 242. In the conventional memory system 250 having an error correction logic, the encoder 242 calculates ECC bits corresponding to the data before writing the data bits plus the ECC bits to the memory array 270. When data are read from the memory array 270, the read data and ECC bits are received by an ECC decoder 232 coupled to the memory array 270 by the read bus 233. The ECC decoder 232 calculates new ECC bits from the read data and compares the new ECC bits to the ECC bits read from the memory array 270. If the ECC bits match, no errors are considered to exist in the read data. If the ECC bits do not match, the ECC bits read from the memory array 270 are used to correct the read data. The read data are then transmitted on to a read latch 234, and then passed on to drivers 238 to be sent out as read data DQ<0:7>.
Conventional error detection methods such as those used in the memory system 250 of FIG. 2, become inadequate when having to test the entire memory array 270. As described previously, every memory cell in the array 270 must be read and checked for data errors during device testing. Checking the data stored in the memory cells requires a substantial period of time primarily because of the time required to generate ECC bits from the read data using the equations shown in FIG. 1B. Once the ECC bits have been generated, they must be compared to the ECC bits read from the memory array 270, which are used to correct any data errors that are found, as previously explained. As the number of memory cells in memory arrays is now very large and continues to increase, a substantial amount of the time is required to test all of the memory cells in the array 270.
Therefore, there is a need for an alternative approach to using ECC systems in order to reduce testing time when testing a large amount of data, such as data read from the memory cells of a large memory device or of several memory devices.